1. Field of the Invention
The invention relates to intrinsic safety barriers and more particularly to intrinsic safety barriers which are adapted to unidirectional transmission of alternating electrical signals without substantial distortion.
2. Description of the Prior Art
Intrinsic safety barriers have been available for many years. They were originally employed with signaling equipment in underground mines to enable the emission of intended electrical signals from the mine and to prevent introduction of electrical energy into the mine through the signaling system.
Recently the National Fire Protection Association has issued standards for intrinsically safe process control equipment, NFPA No. 493. The theory of intrinsic safety devices is that a minimum quantity of electrical energy is required to ignite or detonate explosive gases. When an electrical system is intrinsically safe, that minimum amount of electrical energy cannot be introduced into the system.
There are presently available intrinsic safety barriers which are suited to the protection of direct current systems. Such existing systems include a single Zener diode connected across the input terminals of the safety barrier circuit. The single Zener diode presents low electrical resistance in one direction and high electrical resistance in the opposite direction. If the applied voltage across a Zener diode is increased beyond a breakdown voltage level, the Zener diode exhibits low resistance in both directions. When an alternating electrical signal is applied to the input terminals of such prior art safety barrier circuits, the output signal becomes unsymmetrical, i.e., distorted, unless the amplitude of the input electrical signal is maintained at a relatively low value.